1. Technical Field
The present invention relates to an optical filter for measuring the characteristic of an incident light, characteristic measurement method using the optical filter, an analytical instrument equipped with the optical filter, and an optical device equipped with the optical filter.
2. Related Art
In the past, there has been known a variable wavelength interference filter provided with a pair of reflecting films disposed so as to be opposed to each other with a gap therebetween, and for dispersing the light with a desired wavelength from the incident light by varying the size of the gap (see, e.g., JP-A-11-142752 (Document 1)).
The variable wavelength interference filter described in Document 1 is provided with a pair of substrates, and a pair of multilayer films (reflecting films) disposed on the respective surfaces of the substrates opposed to each other. Further, electrostatic drive electrodes are formed respectively on the pair of reflecting films of variable wavelength interference filter, and it is arranged that the distance between the gaps can be adjusted by applying a voltage to the electrostatic drive electrodes.
Incidentally, in a spectral measurement device equipped with the variable wavelength interference filter as described in Document 1 mentioned above, the voltage applied to the electrostatic drive electrodes of the variable wavelength interference filter is switched to thereby switch the wavelength of the light to be dispersed, and then the intensity of the light thus dispersed is measured. On this occasion, there arises a problem that when switching the voltage to be applied to the electrostatic drive electrodes from a high voltage to a low voltage, it takes much time for the gap spacing of the pair of reflecting films to reach the desired value. In other words, an electrostatic attractive force F caused by the electrostatic drive electrodes is expressed by a function of the applied voltage and the distance (an opposed electrode distance) between the pair of electrostatic drive electrodes as shown in Formula 1 below.
                              F          ⁡                      (                          x              ,              V                        )                          =                              α            ⁡                          (                              V                                  g                  -                  x                                            )                                2                                    (        1        )            
Here, in Formula 1, “V” denotes the applied voltage to the electrostatic drive electrodes, “g” denotes the opposed electrode distance in the initial state with no voltage applied, “x” denotes a relative displacement amount of the pair of electrostatic drive electrodes, and “α” denotes a constant. As expressed by Formula 1 described above, since the electrostatic attractive force F is inversely proportional to the opposed electrode distance (g−x)2, the larger the opposed electrode distance is, the weaker the electrostatic attractive force F becomes. Therefore, in the case of switching the voltage to be applied to the electrostatic drive electrodes from a high voltage to a low voltage to thereby make the electrostatic attractive force F act so as to increase the opposed electrode distance, the electrostatic attractive force F suddenly becomes weak at the switching timing of the voltage. On the other hand, a restoring force for restoring the substrate to the initial state acts on the substrate provided with the electrostatic drive electrode as a reactive force of the electrostatic attractive force F when applying the high voltage. Therefore, when switching the voltage to be applied to the electrostatic drive electrodes from the high voltage to the low voltage, the restoring force becomes stronger than the electrostatic attractive force F at the switching timing, and there occurs a so called overshoot, namely the fluctuation of the gap spacing exceeding the desired value. Since a free damped vibration is caused in the substrate provided with the electrostatic drive electrode if such an overshoot occurs, there arises a problem that the measurement is not achievable until the fluctuation in the gap spacing is eliminated, which hinders prompt spectral measurement.